What causes the seagull deformity on hand radiography?
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Answer:
The seagull or gull-wing deformity is the appearance of an interphalangeal joint in which central erosion of the proximal articular surface combines with peripheral bony proliferation, so the concave central defect flanked by upturned marginal spurs resembles a gull in flight. It is the hallmark of erosive osteoarthritis, an aggressive inflammatory variant of hand osteoarthritis that affects the distal more than the proximal interphalangeal joints, typically in postmenopausal women, and spares the metacarpophalangeal joints and wrist. The differential is the key to reporting it. Psoriatic arthritis produces marginal rather than central erosions with adjacent proliferative new bone, fluffy periostitis, the pencil-in-cup deformity and involvement of the whole digit with dactylitis and nail disease. Rheumatoid arthritis produces marginal erosions with periarticular osteopenia and uniform joint space loss at the metacarpophalangeal and proximal interphalangeal joints and the ulnar styloid, and characteristically spares the distal interphalangeal joints. Gout produces well-corticated para-articular erosions with overhanging edges and preserved joint space with soft tissue tophi. Erosive osteoarthritis may end in ankylosis of the affected joints, which rarely happens in nodal osteoarthritis, and MRI or ultrasound demonstrate the synovitis that explains the inflammatory clinical presentation.
Why is it called so?
The central depression with raised edges on both sides looks like the outstretched wings of a gull.
Pathophysiology
In erosive osteoarthritis the cartilage of the interphalangeal joint is lost and the subchondral bone plate collapses centrally, producing a saucer-shaped defect in the head of the proximal phalanx. At the same time the joint margins respond to abnormal mechanical loading by producing osteophytes, so bone is being lost centrally and formed peripherally at the same joint. The base of the adjacent distal phalanx remodels into a corresponding convex shape that fits into the defect, which is the cause of the associated instability. Superimposed synovial inflammation, with a cellular infiltrate more marked than in ordinary osteoarthritis, drives the erosive component and explains the pain, swelling and the tendency of some joints to fuse.
Alternative names: Gull-wing deformity; gull wing sign
Other associated named signs: Pencil-in-cup deformity of psoriatic arthritis and the rat bite erosions of gout
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